ENC28J60-CSLASHSP datasheet(8/96 Pages) MICROCHIP

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ENC28J60

DS39662B-page 6

Preliminary

2.3

CLKOUT Pin

The clock out pin is provided to the system designer for

use as the host controller clock or as a clock source for

other devices in the system. The CLKOUT has an

internal prescaler which can divide the output by 1, 2,

3, 4 or 8. The CLKOUT function is enabled and the

prescaler is selected via the ECOCON register

(Register 2-1).

To create a clean clock signal, the CLKOUT pin is held

low for a period when power is first applied. After the

Power-on Reset ends, the OST will begin counting.

When the OST expires, the CLKOUT pin will begin out-

putting its default frequency of 6.25 MHz (main clock

divided by 4). At any future time that the ENC28J60 is

reset by software or the RESET pin, the CLKOUT func-

tion will not be altered (ECOCON will not change

value). Additionally, Power-Down mode may be

entered and the CLKOUT function will continue to

operate. When Power-Down mode is cancelled, the

OST will be reset but the CLKOUT function will

continue. When the CLKOUT function is disabled

(ECOCON = 0), the CLKOUT pin is driven low.

The CLKOUT function is designed to ensure that mini-

mum timings are preserved when the CLKOUT pin

function is enabled, disabled or the prescaler value is

changed. No high or low pulses will be outputted which

exceed the frequency specified by the ECOCON

configuration. However, when switching frequencies, a

delay between two and eight OSC1 clock periods will

occur where no clock pulses will be produced (see

Figure 2-3). During this period, CLKOUT will be held

low.

FIGURE 2-3:

CLKOUT TRANSITION

ECOCON

Changed

80 ns to 320 ns Delay

ECOCON: CLOCK OUTPUT CONTROL REGISTER

U-0

R/W-1

R/W-0

—

COCON2

COCON1

COCON0

bit 7

bit 0

Legend:

R = Readable bit

W = Writable bit

U = Unimplemented bit, read as ‘0’

-n = Value at POR

‘1’ = Bit is set

‘0’ = Bit is cleared

x = Bit is unknown

bit 7-3

Unimplemented: Read as ‘0’

bit 2-0

COCON2:COCON0: Clock Output Configuration bits

11x

= Reserved for factory test. Do not use. Glitch prevention not assured.

101

= CLKOUT outputs main clock divided by 8 (3.125 MHz)

100

= CLKOUT outputs main clock divided by 4 (6.25 MHz)

011

= CLKOUT outputs main clock divided by 3 (8.333333 MHz)

010

= CLKOUT outputs main clock divided by 2 (12.5 MHz)

001

= CLKOUT outputs main clock divided by 1 (25 MHz)

000

= CLKOUT is disabled. The pin is driven low.

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